I am aiming for 99, aiming to get into B Sc Adv Math. From where I stand, you must put hard work into the HSC for a good ATAR. But I don't think you need to slave over 5-6hrs per day in order to achieve this. If you set out long term, mid term and short term, right up to daily, nightly within the hourly goals, then you will get all your work done, with time for revision and practice. It is also knowing how each subject is going to affect your ATAR. Getting 100 in MX2 and 80 in EngAdv will not produce a high ATAR, but a 90 in both will. That's just an example.
The journey is different for everyone, some only need 1-2hrs. Some will need 3-4. But no one should be doing 5-6, IMHO, as this is slave labour. You are obviously doing something wrong if you do this. I'm still in Yr11 but I'm aiming for 2-4hrs a day for yr12.
That's just my opinion.